This invention relates to high temperature superconductors and, more particularly, to enhancement of the upper critical field and current carrying capacity of such materials.
High temperature superconducting ceramics (HTSC) have been discovered in recent years. A typical material such as YBa.sub.2 Cu.sub.3 O.sub.7 (YBCO) is a member of this perovskite family of copper based oxides that combine easily with alkaline and rare earth ions to form these HTSC. These materials hold great promise for industrial and military applications. For optimum utility they should have a current carrying capacity greater than 10.sup.9 amperes/meter.sup.2. This physical property requires the HTSC to have high critical current density (J.sub.c) and high upper critical magnetic field (H.sub.c2).
It is known that such desirable characteristics can be achieved in Type II superconductors which exhibit flux pinning, that is, magnetic flux generated from current flow is fixed in place in the body of the superconductor. Also, in an increasing magnetic field, the entry into the HTSC of significant amounts of flux may not occur because of pinning effects until a field several times the lower critical field (H.sub.c1) has been applied, and when the flux does penetrate, it does so slowly (with respect to the size of the magnetic field) and penetration is incomplete until the applied field equals H.sub.c2.
Investigative attempts are currently underway to enhance flux pinning in HTSC. A first method involves shock-compaction and is described in "Superconducting and Microstructural Properties of Shock-Impacted High-TC Oxide Powders"--C. L. Seaman, M. B. Maple, U. California, San Diego; W. J. Nellis, J. B. Holt, M. Kamegai--Lawrence Livermore Laboratory, presented at 1989 Topical Conference on Shock Compression of Condensed Matter, Alberquerque, N.M., Aug. 14-17, 1989. A second method involves alteration of the width of the grain boundary layer by chemical means and is described in "Grain Boundaries and Critical Current in Type II Superconducters", A. DasGupta, U.S. Department of Energy, Chicago Operations Office, 9800 South Cass Avenue, Argonne, Ill. 60439, presented at The Second International Ceramic Science and Technology Congress and The American Ceramic Society's Electronics Division Meeting, Orlando, Fla., Nov. 12-15, 1990. However, neither of these attempts has achieved entirely satisfactory results.
U.S. Pat. No. 3,149,232 of Jaffe et al discloses a method of irradiating piezoelectric ceramic materials with gamma radiation to improve some of their electrical and mechanical properties. However, there is no suggestion by Jaffe et al that gamma radiation could be used to enhance the uppper critical magnetic field and current carrying capacity of superconducting materials, particularly high temperature superconducting ceramic materials such as copper oxide perovskites.
It is therefore a primary object of this invention to treat HTSC to enhance the upper critical magnetic field (H.sub.c2) therein.